Sunday, 22 March 2015

Pliocene (Pt 4): Before the Mammoths

Anancus arvernensis

One of the markers for the end of the Pliocene is the "Mammoth-Horse Event" when the slide into autumnal temperatures that had defined the later part of the epoch reached the point that significant quantities of ice began to build up in the Northern Hemisphere - notably in Greenland, where it still remains today. In Europe, this cooler weather brought an end to many of the more northerly forests, replacing them with open tundra-like grassland. Many aspects of the local fauna didn't change all that much, although they likely edged a bit further south.

But the arrival of the grasslands did herald the appearance of single-toed horses, and of mammoths - albeit not the woolly ones that most people immediately think of. Mammoths are perhaps the poster-child for the Ice Ages, but Europe was, of course, not devoid of such large animals before their coming.
In the epoch before the Pliocene, Europe had been home to a surprisingly large number of rhinos. Almost all died out in the aftermath of the Zanclean Flood, so that their descendants are today found only in Africa and southern Asia. Most, but not all. One rhinoceros did survive: Stephanorhinus. Initially adapted to browsing on relatively soft vegetation, this did well because, at the time, much of Europe was a much lusher place than it is today (let alone than it would become once the Mammoth-Horse Event arrived). Nonetheless, we do see, by the mid Pliocene, at least one species of the genus, S. miguelcrusafonti, that had higher-crowned teeth than its close relatives, a sign that it ate tougher food; most likely the newly formed Mediterranean scrub of Spain and southern France, where it lived.

The closest living relative of these rhinos is probably the near-extinct Sumatran rhinoceros of south-eastern Asia, rather than any of the African species. But nonetheless, they were something of a success story in Europe for millions of years, living on, in various forms, into the early Pleistocene, where they were finally killed off by the bitter cold of the Ice Ages. Even then, the line itself survived, for they were likely also close relatives of the woolly rhino, which entered Europe from Siberia just as Stephanorhinus was dying out.

Larger even than the rhinos, however, were animals rather more closely related to mammoths. At the dawn of the Pliocene, one of the creatures that survived the Flood and its aftermath was Borson's mastodon, Zygolophodon, a reasonably close relative of the mastodons of North America. It did not, however, survive terribly long, and mastodons did not return to Europe after its demise.

The largest animal in Pliocene Europe, however, was likely Anancus, another elephant-like beast. Anancus was not a true elephant, in that it was not a member of the elephant family, as mammoths were... but quite what it actually was is a little less clear. When the first specimens were discovered, in the 19th century, they were described as mastodons, on the grounds that their teeth had the multiple domed points you would expect for those animals, rather than the parallel ridges of elephant teeth.

As we began to get a rather more detailed picture of the evolution of the elephant-like animals as a whole, Anancus was instead moved to a group called the gomphotheres. Gomphotheres were a large and diverse group of animals that were probably closer to elephants than to mastodons, but were quite clearly neither. Indeed, Anancus does have some features that are gomphothere-like, but it does have some differences, too. The most visible of these is that Anancus had two tusks, just like an elephant... but gomphotheres typically had four. That, in itself doesn't prove anything (mastodons had two tusks, and they aren't elephants), but, taken with other factors it's enough to raise a doubt, and many researchers today consider that Anancus, if not actually members of the elephant family, were at least very close to it.

Physically, Anancus arvernensis, the most common species, stood about the same height as today's Asian elephants. They may actually have weighed more, however, since they had rather shorter legs, meaning that more of that height was made up from the bulk of the main body. There were, of course, other visible differences, but the most apparent one would have been the tusks. They projected more or less straight forward, rather than in the more curved shape that fully grown tusks have in male elephants and mammoths. But, more significantly, they were up to 4 metres (13 feet) long. This, remember, in an animal that's only around 3 metres (10 feet) tall.

This, one can imagine, is the sort of thing you'd notice.

Okay, so that's a maximum, rather than the average, but it still makes one wonder what on Earth it used them for. Presumably, they'd have helped in defending the animal's young against sabretooth cats and whatever else might want to eat them, but it's unlikely that that would be their only purpose. They could also have stripped bark from trees, or been used to root up tubers from the ground, with the greater length perhaps being of particular use in the latter task. Males, of course, could also have fought one another with them, and its perfectly possible that female Anancus just thought big tusks were sexy, developing large tusks of their own as a sort of genetic by-product of repeatedly mating with long-tusked males.

A forest-dwelling animal, the advancing tundra grasslands of the Pleistocene finally drove Anancus to extinction. Nonetheless, it did survive the Mammoth-Horse Event for a while, and lived alongside the first of the mammoths, the southern mammoth (Mammuthus meridionalis). This was a particularly large animal, perhaps slightly larger than a modern African elephant. It already had the dramatically curved tusks that are typical of mammoths, although, having originated from African ancestors, and living in open woodland and mixed pasture, rather than out on the tundra proper, it likely wasn't anywhere near as hairy as its more famous descendant. With the domed head of true elephants, in life the shape of the tusks might well have been a time traveller's only clear indication that it wasn't one of the living species.

During the Salinity Crisis of the late Miocene, animals could easily cross from Africa into Europe by the simple expedient of walking from Morocco to southern Spain (and that's assuming that they avoided the more hazardous route of crossing the sweltering hot salt flats of the former Mediterranean basin). Many of the northern populations resulting from this migrations died out after the Flood, but elephants, rhinos, and antelope were by no means the only creatures to survive - there were also monkeys in Pliocene Europe.

The most common was a kind of macaque so similar to those we have today that it may well actually have been the same species as the living Barbary macaque (Macaca sylvanus). It lived from France across to at least Hungary, and even survived through most of the Ice Ages, perhaps suffering more from the eventual arrival of humans than it actually did from the weather. Assuming that the fossil form is the same species as the living one, it survives to the present day in northwestern Africa, and a colony of the animals were even re-introduced to Gibraltar in 1740, where they still live wild today.

One European monkey of the time was, however, a bit stranger. This was Paradolichopithecus, and it's unusual in that it doesn't appear to have done a lot of climbing. Judging from its body shape, it's thought that it would have walked along the ground on all fours, presumably foraging in relatively open terrain. Taken together with its huge size for a monkey (perhaps as much as 35 kg or 80 lbs), and its unusually long snout, it's hard to escape the conclusion that this was some kind of baboon.

Except it wasn't. We can tell, from detailed study of the internal structure of the skull, that this, too, was a macaque. Or, at least, closer to a macaque than anything else alive today. In fact, it probably snuck into Europe from Asia, not Africa. While it is true that the baboons themselves are descended from a macaque-like monkey that descended to the ground, the existence of Paradolichopithecus demonstrates that the same evolutionary event occurred more than once, presumably under similar pressures, but on different continents.

In fact, macaques weren't the only monkeys in Europe at the time, for there were also colobus monkeys, a group that (at least in living forms) are highly adapted to eating leaves. Many of these were broadly similar to the modern kind, but it's interesting to note that one, Dolichopithecus, was, like the species that shares most of its scientific name, better suited to the ground than the trees. Having said which, possibly not quite as much as the macaque in question, and its teeth do suggest that it mostly ate leaves, but still, living on the ground is a rather stranger thing for a colobus monkey to do than it is for a macaque.

It may be that these monkeys came down from the trees partly to escape their usual predators - the unusually large size of Paradolichopithecus would help in that regard too, of course. But then, Pliocene Europe wasn't exactly free of predators, and it is to them that I will turn in Part Five...

2 comments:

re:"(mastodons had two tusks, and they aren't elephants)"Well,most pictures of the classic American Mastodon only show two tusks (in the upper jaw, like the tusks of living elephants), but it (like elephants) was descended from four-tuskers. In fact, the A.M. had vestigial tusks in the lower jaw. The mounted skeleton in the American Museum of Natural History (New York) is missing one, but the other is still in its socket. I haven't been there for a few years, and I wouldn't put much faith in my memory of dimensions, but I'd guess that it is about an inch and a half (ca. 40mm) in diameter with maybe six inches (ca 15cm) of its length projecting beyond the surrounding jawbone. How much would have been visible in life I don't know: maybe it would have been hidden behind the lip most of the time. But it's one of my favorite specimens from the fossil mammal hall because of its illustration of the evolutionary history of two-tuskedness in the mastodon and elephant lineages.